C. J. CARLILE AND B. T. M. WILLIS 715 We do not envisage that the present technique will be restricted to the determination of sound velocities. It can be used also to study the nature of acoustic vibrations in the vicinity of phase transitions, or to investigate excitations away from the origin of the Brillouin zone. These are the types of problem to be examined in future publications. We thank Dr W. I. F. David and Mr R. M. Ibberson for their help in carrying out the neutron experiments on HRPD. We are also grateful to Professor A. Albinati for useful discussions. References EWALD, P. P. (1919). Phys. Z. 14, 465-472. GERLICH, D. (1964). Phys. Rev. 135, A1331-A1333. JOHNSON, M. W. & DAVID, W. I. F. (1985). Rep. RAL-85-112. Rutherford Appleton Laboratory, Didcot, Oxon, England. LOWDE, R. D. (1954). Proc. R. Soc. London Set. A, 221,206-223. SCHOFIELD, P. & WILLIS, B. T. M. (1987). Acta Cryst. A43, 803-809. SEEGER, R. J. & TELLER, E. (1942). Phys. Rev. 62, 37-40. WALLER, I. & FROMAN, P. O. (1952). Ark. Fys. 4, 183-189. WILLIS, I. T. M. (1970). Acta Cryst. A26, 396-401. WILLIS, B. T. M. (1986). Acta Cryst. A42, 514-525. WILLIS, B. T. M., CARLILE, C. J., WARD, R. C., DAVID, W. I. F. & JOHNSON, M. W. (1986). Europhys. Lett. 2, 767-774. Acta Cryst. (1989). A45, 715-718 The Use of MULTAN to Locate the Positions of Anomalous Scatterers BY A. K. MUKHER2EE,* J. R. HELLIWELLt:~ AND P. MAIN Department of Physics, University of York, Heslington, York Y01 5 DD, England (Received 2 November 1988; accepted 19 May 1989) Abstract Observed anomalous scattering differences have been used with the direct-methods program MULTAN87 to determine the positions of anomalous scatterers in a variety of metalloproteins and a small molecule. The lack of anomalous differences in the centric data did not prevent the determination of the atom posi- tions and the anomalous scatterers were found in all cases. These results show that the method may be useful to determine the positions of anomalous scat- terers in the case of multi-site genetically engineered proteins. Introduction Anomalous scattering measurements are increasingly being used in macromolecular crystal structure analy- sis because of the availability of synchrotron radiation. The variable wavelength makes avail- able multiple-wavelength anomalous-dispersion data from which structures have now been determined by various groups. The technique can be readily applied to the metalloproteins. In addition, it is becoming easier to prepare heavy-atom derivatives and these need not be isomorphous with a native protein if multi-wavelength techniques are used. The location of the metal atoms is required before phases can be calculated. A Patterson synthesis can * Permanent address: Department of Physics, Jadavpur Univer- sity, Calcutta-700032, India. t Present address: Department of Chemistry, University of Manchester, Manchester M13 9PL. ~tAuthor to whom correspondence should be addressed. 0108-7673/89/100715-04503.00 be used with anomalous, isomorphous or combined differences, but the interpretation is straightforward when there are only a few sites. For more than about four sites, interpretation becomes less easy. However, with genetic engineering, the possibility of incor- porating as many as 20 atoms into a molecule is not unreasonable. Direct methods have been used to locate metal atoms in proteins on the basis of isomorphous differences [see, for example, Wilson (1978) and Adams, Helliwell & Bugg (1977)]. Given the prospect of a large number of metal sites and the possibility of using multi-wavelength methods, we decided to explore the use of anomalous differences with the direct-methods program MULTAN87 (Debaer- demaeker, Germain, Main, Tate & Woolfson, 1987) to locate the metal atoms. We have restricted ourselves to the use of the imaginary component (Af") derived differences, i.e. differences measured at one wave- length. These are inherently more accurate than the estimate of Af' from multiple-wavelength experi- ments because of difficulties with absorption correc- tions at the different wavelengths. The results are therefore applicable to both conventional and syn- chrotron X-ray sources. Method Anomalous differences can be expressed (Blundell & Johnson, 1976) as Aano = F+I-IF-[ -~ 2F~no cos (~o- ~0~no) = 2F~.o cos A~ (1) where the symbols are defined in Fig. 1. O 1989 International Union of Crystallography